The present invention relates to sulphonic fluorinated ionomers crosslinked by radical route suitable for the preparation of membranes for electrochemical applications, in particular for fuel cells, and as ionic exchange resins acting as catalysts.
Specifically, the invention relates to sulphonic fluorinated ionomers crosslinked by radical route characterized by a high hydration degree, both at room temperature and at high temperature (up to 180xc2x0 C.), without substantially compromising the physical integrity of the obtained membranes, wherefore they are usable also at high temperatures in the range of 120xc2x0 C.-180xc2x0 C.
It is known in the prior art the use of the class of polymers called by the term xe2x80x9cionomersxe2x80x9d in electrochemical applications, such for example in fuel cells, chlorosoda cells, lithium batteries and in reactors in which the ionomer acts as a solid catalyst. These applications implies the contact of the ionomer with a liquid, in particular water, having affinity with the ionic functional groups of the ionomer itself.
Generally, the larger the amount of ionic groups present in the chain, the better the efficiency of the ionomer application, both in terms of capability of ionic exchange in electrochemical applications, and in terms of catalyst activity in catalysis applications. From this point of view, an important parameter is the equivalent weight of this ionomer. The lower the equivalent weight, the higher the percentage of sulphonic groups present in the chain. Therefore, ionomers having a low equivalent weight are desirable since they give a higher application efficiency.
In electrochemical applications, for example in fuel cells, there is a direct correlation between the polymer conductivity and the water retention from the ionomer itself. The ionic conductivity of the polymer, besides being increased by the greater presence of ionic groups in the polymer, results increased, within an upper limit, also by the larger amount of water that the polymer is capable to retain (swelling degree). However, the excessive affinity of the ionomer with water has as a consequence the drawback of an excessive polymer swelling, which assumes a gelatinous state consequently losing its physical integrity. The ionomer becomes therefore completely unusable in all the applications wherein it is required under a solid form.
Also in the applications wherein the ionomer is mixed with or deposited on a support material, suitable to guarantee the shape and the physical integrity of the final membrane, the ionomer must however show a physical consistency sufficient to prevent the release thereof from the support and it must be insoluble in the liquid medium with which it comes into contact during the use.
Besides, the ionomeric membrane must be activated before the use, wherefore the chemical transformation of the precursor groups xe2x80x94SO2F into the corresponding sulphonic groups xe2x80x94SO3H is necessary. The membrane activation is carried out first by contacting it with an alkaline aqueous solution and then with an acid solution. During this transformation phase, if the ionomer has a high swelling degree, it can partially or completely dissolve in the reaction medium. At this point, it is impossible to recover the ionomer and separate it from the other products of the transformation reaction.
In the prior art, to obtain a limited ionomer hydration and sufficient physical integrity, polymers having a high equivalent weight, of the order of 1,000-1,200, are used i.e. having a low concentration of sulphonic groups, which represent the hydrophilic part of the polymer. Therefore, ionomers having a high equivalent weight absorb a limited amount of water, which guarantees the polymer insolubility. On the other hand, having few ionic groups, they have the drawback to give membranes having a lower ionic conductivity during the application. An example of said ionomers is represented by the commercial product NAFION(copyright), used in fuel cells and having an equivalent weight of the order of 1,000-1,100. The membranes obtained from said ionomers have good mechanical properties. However, if these membranes are used at temperatures higher than 100xc2x0 C., the interstitial water, which is the carrier of the protons H+ in fuel cells, tends to reduce itself, wherefore the membrane tends to dehydrate and the membrane conductivity is drastically reduced. As a consequence, the membranes obtained by NAFION(copyright) are not efficiently usable at temperatures higher than 100xc2x0 C.
U.S. Pat. No. 4,940,525 describes sulphonic ionomers having a low equivalent weight, lower than 725, used to obtain unsupported thick membranes for fuel cells, only if the hydration product of the polymer is low, lower than 22,000. So low hydration values are indeed necessary for maintaining the physical integrity of the ionomer having equivalent weights lower than 725, provided that the equivalent weight is not lower than 500 (col. 6, 8-16). Therefore, according to the description of this patent, it is impossible to obtain sulphonic ionomers of equivalent weight lower than 500 having the property of the insolubility in water. Besides, no mention is made to the behavior of the membranes at high temperatures, of the order of 120xc2x0 C.-160xc2x0 C.
The need was felt to have available sulphonic fluorinated ionomers such that the obtained membranes have a high hydration percentage, both at room temperature and at high temperature (up to about 180xc2x0 C.) without substantially compromising the membrane physical integrity, wherefore the membranes are usable also at high temperatures, of the order of 120xc2x0 C.-180xc2x0 C., in electrochemical applications. Said membranes can be used also as ionic exchange resins.
An object of the present invention are therefore crosslinked sulphonic fluorinated ionomers obtainable by radical crosslinking of:
A) crosslinkable sulphonic fluorinated ionomers, having equivalent weight 380-1300 g/eq, preferably 380-800 g/eq, and comprising:
from 48% to 85% by moles of monomeric units deriving from tetrafluoroethylene (TFE);
from 15% to 47% by moles of fluorinated monomeric units containing sulphonyl groups xe2x80x94SO2F;
from 0.01% to 5% by moles of monomeric units deriving from a bis-olefin of formula: 
xe2x80x83wherein: m=2-10, preferably 4-8;
R1, R2, R5, R6, equal to or different from each other, are H or C1-C5 alkyl groups;
B) a fluorinated compound as crosslinking radical initiator;
C) a fluorinated bis-olefin of the above structure (I) as crosslinking agent;
the radical crosslinking being carried out at a temperature in the range 250xc2x0 C.-310xc2x0 C., preferably 260xc2x0 C.-300xc2x0 C.
Among the fluorinated monomers containing sulphonyl groups xe2x80x94SO2F we can mention:
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F;
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94[CF2xe2x80x94CXFxe2x80x94O]nxe2x80x2xe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F wherein Xxe2x95x90Cl, F or CF3; nxe2x80x2=1-10;
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F (vinylsulphonylfluoride);
F2Cxe2x95x90CFxe2x80x94Arxe2x80x94SO2F wherein Ar is an aryl ring.
Preferably the crosslinkable fluorinated sulphonic ionomers A) comprise:
from 54% to 71% by moles of monomeric units deriving from TFE;
from 45% to 28% by moles of monomeric units deriving from vinylsulphonylfluoride F2Cxe2x95x90CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F;
an amount higher than 0.4% by moles up to 3% by moles, more preferably from 1% to 2.5% by moles of monomeric units deriving from the bis-olefin of formula (I).
As regards the radical initiators B) used in the radical crosslinking of the present invention, they are selected from:
(d)-branched perfluoroalkanes of formula: CaF2a+2 wherein a=5-15, preferably 7-11;
(e)-halogenated compounds of formula: ClO2S(CF2)nSO2Cl wherein n=4-10;
(f)-peroxidic perfluoropolyether compounds having oxidizing power in the range 0.8-6, preferably 1-3.5, of structure
Txe2x80x94Oxe2x80x94(Rf)xe2x80x94(O)cxe2x80x94Txe2x80x2
wherein:
T,Txe2x80x2xe2x95x90xe2x80x94CF3,xe2x80x94CF2CF3, xe2x80x94CF2CF2CF3
c is an integer such as to give the above oxidizing power;
Rf perfluoropolyether chain having a number average molecular weight in the range 1,000 and 30,000, preferably 4,000-20,000, comprising one or more of the following units:
xe2x80x94(CF2O)xe2x80x94, xe2x80x94(CF2CF2O)xe2x80x94, xe2x80x94(CF2CF2CF2O)xe2x80x94, xe2x80x94(C3F6O)xe2x80x94
Preferably the perfluoropolyether chain Rf is selected from the following structures:
xe2x80x94(CF2O)axe2x80x2(CF2CF2O)bxe2x80x2xe2x80x94
with bxe2x80x2/axe2x80x2 in the range 0.1-40, preferably 0.5-20, axe2x80x2 and bxe2x80x2 being integers such as to give the above molecular weight;
xe2x80x94(CF2O)cxe2x80x2(C3F6O)dxe2x80x2xe2x80x94
with cxe2x80x2/dxe2x80x2 in the range 0.01-5, cxe2x80x2 and dxe2x80x2 being integers such to give the above molecular weight.
As oxidizing power it is meant the amount in grams of peroxidic oxygen per 100 grams of polymer.
As radical initiator, a peroxidic compound of formula (f) with the above oxidizing power is more preferably used. The preparation of the peroxidic initiators (f) can be carried out according to the processes described in U.S. Pat. Nos. 3,847,978 and 5,488,181.
It has been found by the Applicant that both in the case of too low oxidizing power and in the case of too high oxidizing power, the crosslinking of the ionomeric chains is poor. The Applicant has found that using as initiator the compound of formula (f) having an oxidizing power within the above values, a very good level of crosslinking is obtained.
When a radical initiatior of formula (f) is used for the crosslinking, it is fed in a concentration in the range 2%-10% by weight, preferably 3-6% by weight with respect to the weight of the sulphonic ionomer.
As regards the crosslinking agent bis-olefin of formula (I), it is fed in a concentration in the range 3%-25% by weight, preferably 4-8% with respect to the weight of the sulphonic ionomer.
According to a not binding theory, the Applicant keeps that in the crosslinking initial phase, the radical initiator has the function to remove a proton from the bis-olefin of formula (I) present in the chain of the sulphonic ionomer. The so formed radical can attack the double bond of the bis-olefin c) introduced as crosslinking agent and generate the first part of the reticule. The repetition of this H* extraction event on another ionomeric chain allows the reticule formation.
A further object of the invention are the supported or self-supported membranes, and their process of preparation by using the crosslinked sulphonic fluorinated ionomers of the present invention.
For supported membranes, foamed PTFE, preferably bistretched, can be used as support, having a suitable dimension in connection with the electrochemical cell, having a thickness comprised between 10 xcexcm and 50 xcexcm. For the preparation of supported membranes, first a solution is prepared formed by:
A) a solution of the sulphonic ionomer in a fluorinated solvent selected from: hexafluoroxylene, perfluorohexane, perfluorooctane, perfluorobenzene, perfluoropolyether solvents, fluoroether solvents.
B) a radical initiator of formula (d), (e) or (f);
C) the bis-olefin of formula (I) as crosslinking agent.
The porous support of foamed PTFE is dipped in the solution formed by A)+B)+C) for a time comprised between 10 seconds and 1 minute; the impregnation is repeated more times until an impregnated membrane is obtained. Subsequently the ionomeric solution excess is removed from the support, for example by a roller system. The supported membrane obtained by impregnation is then dried at 25xc2x0 C. for about 1 hour, to remove the fluorinated solvent.
At this point, the membrane is crosslinked at the above mentioned temperature, for a reaction time equal to 6 half-lives of the used initiator, i.e. until a decomposition of about 99% of the initiator. When a radical initiator of formula (f) is used, the crosslinking average time generally ranges from about 10 seconds to 3 minutes.
Alternatively, the preparation of the supported membrane can be carried out by casting. In the case of thick membrane (self-supported), the membrane can be obtained by casting or by press. In the latter case a film of the solution A)+B)+C) is placed between two metal plates containing a metal frame, which confers to the resulting membrane the desired thickness. Then, the plate/frame/film system is put in a press oven applying a weight of about 500-2,000 kg, preferably 750-1,500 Kg.
The supported or thick membranes, obtained from the sulphonic ionomers crosslinked by the method of the present invention have a thickness ranging from 10 xcexcm to 300 xcexcm depending on the used preparation method. The membrane appears transparent, in some cases light brown-colored. The morphological analysis at the microscope of the obtained membrane shows a substantially uniform, smooth and completely free from holes surface.
The obtained membrane, when it appears light brown-colored, can be decolorated. Decoloration can be carried out by dipping the membrane in the xe2x80x94SO2F form into an aqueous solution containing H2O2 at 9% (weight/volume) and HNO3 at 10% (weight/volume) at 50xc2x0 C. for about 4-6 hours. A colorless and transparent membrane is obtained with a maximum weight loss lower than 1% by weight (color loss higher than 95% by UV analysis). The decoloration treatment can be carried out both before and after the membrane activation treatment described hereunder.
The membrane is then subjected to the activation treatment for transforming the sulphonyl groups xe2x80x94SO2F into sulphonic groups xe2x80x94SO3H. The activation implies 2 steps:
salification for transforming the xe2x80x94SO2F form into the xe2x80x94SO3xe2x88x92 form;
acidification for transforming the xe2x80x94SO3xe2x88x92 form into the xe2x80x94SO3H form.
Salification is carried out by dipping the membrane obtained after the crosslinking reaction in a basic aqueous solution of KOH or of NaOH at a temperature such as to have an almost total conversion into the xe2x80x94SO3xe2x88x92 groups.
It has been found by the Applicant that, for equivalent weights lower than 800, a temperature in the range 5xc2x0 C.-40xc2x0 C., for a time comprised between 4 and 40 hours, can be used. The 97% of the conversion takes place in the first 20-30 minutes of reaction; further 5 hours are necessary for obtaining the required conversion of 99.9% (upper limit of instrumental detection). The weight loss due to the dissolving of the salified membrane in water is lower than 20%. Temperatures higher than 40xc2x0 C., shorten the conversion times, but they remarkably increase the weight loss of the membrane (for example operating at 70xc2x0 C., there is a weight loss equal to 90-95%). Therefore, the crosslinked membranes obtained from the sulphonic fluorinated ionomers having a low equivalent weight of the invention show the additional advantage to be salified at room temperature.
At the end of the salification, the membrane is dipped into a distilled water bath at 25xc2x0 C. for washing the residual base.
The acidification is carried out by dipping the salified membrane in an aqueous solution containing the 20% by weight of HCl at 25xc2x0 C. for 5 hours. The conversion is equal to 99.9% 20 with a weight loss of the membrane in the xe2x80x94SO3H form lower than 1%. The resulting crosslinked, optionally decolorated, membrane in the xe2x80x94SO3H form, is suitable to be used in applications of electrochemical type, for example of fuel cell type or in catalysis applications as ionic exchange resin.
The crosslinked membranes of the invention after activation, i.e. in the xe2x80x94SO3H form, show the following propeties:
amount of gels higher than 85%, generally higher than 90%;
after drying and hydration in water at 100xc2x0 C. for 30 minutes, the membrane substantially remains integer without dissolving in water.
As xe2x80x9cgelxe2x80x9d is meant the insoluble part of the polymer and the % by weight of gels is correlated with the crosslinking degree of the ionomer itself. Indicatively, the higher the % by weight of gels, the lower the amount of uncrosslinked ionomer.
Tests carried out by the Applicant (see the Examples) show that the obtained membranes, notwithstanding the high hydration percentage, have a good physical integrity and they are substantially insoluble in water both at low and at high temperature (up to about 180xc2x0 C.). In particular, the Tr values (interstitial water release temperature) obtained by thermogravimetric analysis TGA, result surprisingly high.
The interstitial water is the carrier of the H+ protons in fuel cells, wherefore the invention membranes, having a Tr in the range 160xc2x0 C.-180xc2x0 C., are capable to mantain a good conductivity also at very high temperatures, for example of the order of about 160xc2x0 C. This is surprising and unexpected since the membranes of sulphonic ionomers known in the prior art are not usable in an efficient way at temperatures higher than 100xc2x0 C.
Besides in the preparation of membranes for fuel cells, the sulphonic ionomers of the present invention can successfully be used in the preparation of ionic exchange resins for carrying out chemico-physical separations and as acid catalyst for chemical reactions. The crosslinked ionomer of the invention, when used as an acid catalyst, appears extremely effective since it allows to obtain high yields in short times.
As regards the preparation of crosslinkable sulphonic fluorinated ionomers, it can be carried out by polymerization in aqueous emulsion according to well known methods of the prior art, in the presence of radical initiators (for example, alkaline or ammonium persulphates, perphosphates, perborates or percarbonates), optionally in combination with ferrous, cupric or silver salts, or other easily oxidizable metals. In the reaction medium also surfactants of various type are usually present, among which the fluorinated surfactants of formula:
Rfxe2x80x94Xxe2x88x92M+
are particularly preferred, wherein Rf is a C5-C16 (per)fluoroalkyl chain or a (per)fluoropolyoxyalkylene chain, Xxe2x88x92 is xe2x80x94COOxe2x88x92 or xe2x80x94SO3xe2x88x92, M+ is selected from: H+, NH4+, alkaline metal ion. Among the most commonly used we remember: ammonium perfluorooctanoate, (per)fluoropolyoxyalkylenes ended with one or more carboxylic groups, etc.
When the polymerization is over, the ionomer is isolated by conventional methods, such as coagulation by addition of electrolytes or by cooling.
Alternatively, the polymerization reaction can be carried out in bulk or in suspension, in an organic liquid wherein a suitable radical initiator is present, according to well known techniques.
The polymerization reaction is generally carried out at temperatures in the range 25xc2x0 C.-150xc2x0 C., under pressure up to 10 MPa.
The preparation of the sulphonic fluorinated ionomers of the invention is preferably carried out in aqueous emulsion in the presence of an emulsion, dispersion or microemulsion of perfluoropolyoxyalkylenes, according to U.S. Pat. No. 4,789,717 and U.S. Pat. No. 4,864,006.
The present invention will be now better illustrated by the following embodiment Examples, which have a merely indicative purpose but not limitative of the scope of the invention itself.